Generally, nuclear rods comprise a tubular sheath containing pellets of nuclear fuel which is sealed at its extremities by a bottom plug and a top plug. These plugs are welded to the sheath by circular weld beads ensuring a seal between the sheath and the plugs.
In addition to this, the upper plug has a channel for filling the sheath with a pressurized inert gas, for example helium. When the fuel rods have been manufactured, this channel is leaktightly sealed by a spot weld. The circular weld beads may show various types of defects. Thus, porosity may be present. This comprises voids in the weld bead. The weld beads may also show lack of penetration, that is to say parts where the radial extent of the beads is less than desired.
Where a TIG (Tungsten Inert Gas) welding method is used, the weld beads may also have tungsten inclusions. The latter type of defect may also be observed in the spot weld plugging the filling channel.
In addition to this, it may happen that this spot weld is in a position and of such a size that it does not satisfactorily block the filling channel. Lack of penetration in the weld bead and poor positioning of the spot weld are likely to give rise to leaks in the fuel rod and therefore to the release of radioactive particles or gases into the nuclear reactor's primary circuit. Porosities and inclusions have an adverse effect on the mechanical integrity of the rod. These defects have to be avoided and for this reason non-destructive testing procedures are applied to the manufactured fuel rods.
In order to check the quality of the weld beads and the sealing spot welds a digital radiographic image of the corresponding areas of rods undergoing inspection is acquired.
The image acquired is then compared with a stored reference image. This reference image has been obtained from a fuel rod which is considered to be free from any defects. The reference image is therefore subtracted from the image obtained, and then the resulting image is digitally processed in order to cause any defects to appear more clearly, and an overall score is allocated to the weld bead, one score for the sealing spot weld and one score for tungsten inclusions. Beyond threshold values, the rod is considered to be non-conforming.
Because of uncertainties in the positioning of the rods and manufacturing tolerances, the position of a rod and its dimensions on the image obtained differ from those of the rod in the reference image. This therefore means that complex processing of the image obtained by subtraction is required in order to be able to detect any manufacturing defects. Furthermore, it has been found that the reliability of this check has remained low, despite this processing of the image obtained by subtraction, which has made it necessary to reject a large number of rods as being non-conforming in order to be on the safe side.
One objective of the invention is to solve this problem by providing a more reliable method for the non-destructive testing of an element for a nuclear reactor.
With this aim the objective of the invention is a method for the non-destructive inspection of an element for a nuclear reactor, a method comprising the steps of:                a) acquiring a digital radiographic image of at least one area of the element,        b) creating a reference image by digital processing of the image obtained, and        c) comparing the image obtained, which may have been processed, with the reference image to detect the presence of any defects.        
In accordance with particular embodiments, the method may comprise one or more of the following features taken in isolation or in any technically possible combination:                                    the element is part of a nuclear fuel assembly,            the element is a nuclear fuel rod extending along a longitudinal axis and comprising a sheath sealed off by top and bottom plugs and containing a nuclear fuel,            the area comprises the weld bead between one of the plugs and the sheath,            the area comprises a spot weld sealing a channel passing through a plug,            step b) comprises a substep b1) of opening or closing the image by the addition of a structuring element,            the structuring element has an elongated shape along the longitudinal axis of the rod,            the structuring element is a segment of p pixels, p being a whole number which is not zero,            a defect which has to be detected is porosity or lack of penetration in the weld bead, and substep b1) is a substep of opening the image by addition of the structuring element,            a defect which has to be detected is a tungsten inclusion, and substep b1) is a substep of closing the image by addition of the structuring element,            step b) comprises a substep b0) of smoothing the image through a convolver, prior to substep b1),            the convolver is a square of side n pixels, n being a whole number which is not zero,            step c) comprises a substep c1) of calculating the difference between the image obtained, processed if necessary, and the reference image, and dividing that difference by the image obtained, processed if necessary, or by the reference image,            after substep c1) the method comprises a substep c2) of multiplying the image by a coefficient substantially corresponding to the maximum light intensity of the viewing device used to acquire the radiographic image,            after substep c1) the method comprises a substep of smoothing the image through a convolver,            the convolver is a square of side q pixels, q being a whole number,            the convolver lies with its length transverse to the longitudinal axis,            after substep c1) the method comprises a step of binarising the image,            as the defect which has to be detected is a lack of seal at the spot weld, step b) comprises a substep b1) of projecting the image along the longitudinal axis and reconstructing the image from its projection along that axis,            prior to substep b1) step b) comprises a substep b0) of smoothing the image acquired by a convolver,            the convolver is a square of side t pixels, t being a whole number,            step c) comprises a substep c1) of subtracting the reference image from the image obtained, which may have been processed,                        
after substep c1) the method comprises a substep of binarising the image,                                    the method comprises a step d) of automatic detection and determination of the characteristics of a region of the image produced in step c) corresponding to a defect,            one of the characteristics is the position of the defect detected in the image,            one of the characteristics is representative of the dimension of the defect detected,            the method comprises a step d) of determining the minimum axial thickness of the spot weld,            steps a) to d) are performed for several viewing angles,            the method comprises a step e) of reconstructing the defects detected in the images corresponding to different viewing angles,            step e) comprises a substep e1) of determining the positions which a defect detected in a first step corresponding to a first viewing angle may occupy in a second image corresponding to a second viewing angle,            a substep e2) comparing the positions so determined with the positions of the defect(s) actually detected in the second image in order to determine whether the defect has been detected in the second image and if the defect has been detected in the second image,            a substep e3) of calculating a dimension of the defect from the representative characteristics of the dimensions of the defect determined in the first and second images,            the method comprises a step e) of summing the representative characteristics of the dimension determined for several viewing angles and comparing the sum with a threshold value in order to reach a decision on whether the element conforms with predetermined manufacturing criteria,            the method comprises a step e) of calculating a mean for the minimum thicknesses determined for several viewing angles and comparing it with a threshold value in order to reach a decision on whether the element conforms with predetermined manufacturing criteria.                        